Global climate sensitivity derived from ~784,000 years of SST data

Abstract:

Global mean temperatures will increase in response to future increasing greenhouse gas concentrations. The magnitude of this warming for a given radiative forcing is still subject of debate. Here we provide estimates for the equilibrium climate sensitivity using paleo-proxy and modeling data from the last eight glacial cycles (~784,000 years). First of all, two reconstructions of globally averaged surface air temperature (SAT) for the last eight glacial cycles are obtained from two independent sources: one mainly based on a transient model simulation, the other one derived from paleo- SST records and SST network/global SAT scaling factors. Both reconstructions exhibit very good agreement in both amplitude and timing of past SAT variations. In the second step, we calculate the radiative forcings associated with greenhouse gas concentrations, dust concentrations, and surface albedo changes for the last 784, 000 years. The equilibrium climate sensitivity is then derived from the ratio of the SAT anomalies and the radiative forcing changes. Our results reveal that this estimate of the Charney climate sensitivity is a function of the background climate with substantially higher values for warmer climates. Warm phases exhibit an equilibrium climate sensitivity of ~3.70 K per CO₂-doubling – more than twice the value derived for cold phases (~1.40 K per 2xCO₂). We will show that the current CMIP5 ensemble-mean projection of global warming during the 21st century is supported by our estimate of climate sensitivity derived from climate paleo data of the past 784,000 years.